Monitoring and tracking a laterally movable object are important in many applications. In certain applications, it is desirable to have a tracking device not only to monitor the displacement of the object but also to follow the movement of the object without a significant delay. Tracking and following the eye movement during a laser eye surgery is an example of such applications.
Many eye-tracking devices have been developed for eye surgery with lasers, in particular, for photo-refractive surgery. A typical photo-refractive surgery scans an UV laser beam on the cornea to sculpture the profile of the corneal outer surface, one layer at a time. This procedure can correct various refractive disorders of the eye, including nearsightedness, farsightedness, and astigmatism.
Any eye movement during the surgery may adversely affect the outcome of refractive correction. Immobilizing the eye movement during a surgery has been proven difficult in practice. A device automatically tracking and compensating the eye movement is an attractive approach. For the nature of photo-refractive surgery, the tracking device needs to be fast, accurate, and reliable.
U.S. Pat. No. 5,620,436 discloses use of a video camera to monitor the eye's movement and to determine the position of an aiming beam on the eye. U.S. Pat. No. 5,632,742 teaches projecting four laser spots on the eye and using a set of peak-and-hold circuits to determine the position of the eye. In these designs, a ring shape reference is used for determining the eye position, and spatial stationary infrared beams are applied to illuminate the reference. Sophisticated imaging system and electronics, such as a CCD camera or four peak-and-hold circuits are implemented to determine the position of the reference. The ring shape references are practically either the limbus or the iris of the eye and the whole ring is needed as the reference for determining the eye position.